315Dbb0aSfl5SDfl7 


iiii 


•  if  ii: 


liil 


..^..;  _l.>Aj 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 

Boston  Library  Consortium  IVIember  Libraries 


http://www.archive.org/details/fixationofatmospOOsumm 


IMassai 


To  be  presenied  at  the  305th  meeting  of  the 
American  Institute  of  Electrical  Engineers, 
New  York,  March  12,  1915. 

Copyright  1915.     By  A.  I.  E.  E. 

(Subject   to  final   revision  for   the    Transactions.) 


o  ii'  iU 


iA^rSoulttaral 


FIXATION  OF  ATMOSPHERIC  NITROGEN 


BY    LELAND    L.    SUMMERS 


AsbTRACT  OF  Paper 


The  nitrogen  contained  in  the  atmosphere  is  in  an  inert 
form  and  does  not  readily  lend  itself  to  chemical  reactions.  To 
overcome  this  is  the  province  of  "  nitrogen  fixation." 

There  are  very  definite  commercial  limitations  involved 
in  accomplishing  this  as  the  world's  supply  of  nitrogen  has 
been  readily  obtained  from  vast  natural  deposits  of  sodium  nitrate 
in  Peru  and  Chile  and  the  production  of  a  substitute  must  be  at 
a  competitive  cost. 

The  electrical  processes  for  fixing  nitrogen  have  a  very  low 
efficiency,   due  to  utilizing  thermal  energy  only. 

Combinations  of  electrical  and  chemical  methods  promise 
the  most  important  developments. 

Comparative  figures  are  given  showing  amount  of  energy 
necessary  per  kilogram  of  nitrogen  fixed,  and  the  general 
economics  of  the  subject  are  discussed. 

Introductory 
In  1898  Sir  William  Crooks  in  his  address  as  President  of 
■*•  the  British  Association,  very  forcibly  pointed  out  that 
the  commercial  fixation  of  atmospheric  nitrogen  was  one  of 
the  greatest  discoveries  awaiting  the  ingenuity  of  chemists. 
He  emphasized  with  very  interesting  figures  its  important 
practical  bearing  on  the  future  welfare  and  happiness  of  the 
civilized  races.  This  address  brought  forcibly  to  the  attention 
of  engineers  the  fact  that  the  existing  sources  of  fixed  nitrogen 
were  hmited,  and  greatly  stimulated  the  efforts  of  investigators. 
The  problem  itself  had  been  worked  on  for  over  a  century  as 
it  was  known  that  nature  fixed  nitrogen  of  the  atmosphere  by 
means  of  electric  discharges,  and  Cavendish  in  1781  had  shown 
that  a  small  amount  of  nitrogen  was  converted  into  nitric  acid 
in  the  combustion  of  hydrogen  with  oxygen  to  form  water, 
while  Busen  in  1877  obtained  favorable  yields  by  means  of  gas- 
eous explosions.  The  earlier  efforts  commercially  in  the  art 
were  however  largely  confined  to  the  fixation  of  nitrogen  for  the 
purpose  of  manufacturing  cyanides,  and  the  earlier  bibliography 
of  the  subject  therefore  deals  almost  entirely  with  these  efforts. 
Commercial  Products  of  Nitrogen.  The  three  fundamental 
Manuscript  of  this  paper  was  received  January  13,  1915. 

337 


338  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

commercial  products  formed  by  nitrogen  are  first,  its  union 
with  oxygen  to  form  nitrates  NO3  and  nitrites  NO2.  Sec- 
ond, its  union  with  carbon  to  form  cyanogen  C2N2  and 
producing  cyanides  XCN  and  cyanamides  XCN2.  Third, 
its  union  with  hydrogen  to  form  ammonia,  NH3.  From 
all  of  the  above  products  there  are  obtained  a  large  number  of 
derivatives  used  in  the  chemical  arts. 

The  most  important  of  all  commercial  products  are  the  unions 
of  nitrogen  with  oxygen  forming  the  nitric  acid  salts  of  com- 
merce. These  are  of  particular  importance  on  account  of  the 
vast  natural  deposits  of  nitrate  of  sodium  occurring  in  Peru  and 
Chile,  commonly  called  Chile  saltpeter.  Practically,  this  com- 
modity is  the  one  that  sets  the  price  for  all  other  compounds  of 
nitrogen,  as  it  has  been  mined  in  Chile  since  1830,  and  during 
the  past  25  years  its  production  has  assumed  vast  proportions, 
the  present  annual  output  amounting  to  about  2,500,000  tons. 
This  deposit  of  Chile  appeared  inexhaustible  and  therefore  there 
was  no  occasion  for  alarm  regarding  the  world's  supply  of  com- 
bined nitrogen,  but  after  years  spent  in  exploration  work  it  be- 
gan to  appear  that  the  Chilean  deposits  would  be  exhausted 
before  the  end  of  the  present  century,  and  since  then  all  other 
sources  of  combined  nitrogen  have  received  attention. 

While  there  are  a  few  scattered  natural  deposits  other  than 
those  in  Chile,  there  is  none  which  has  at  the  present  time  a 
chance  of  competing,  most  of  them  being  of  limited  extent  and 
situated  in  inaccessible  regions.  In  Chile  the  deposits  are 
easily  worked  and  even  after  years  of  careless  mining  with  no 
effort  to  effect  economies,  the  present  cost  of  producing  nitrate 
is  not  excessive,  varying  from  $10  to  $20  per  ton  and  selling 
in  Liverpool  for  about  $45  per  ton.  This  leaves  a  profit  of  from 
$5  to  $10  a  ton  on  the  operation  after  paying  the  Government 
of  Chile  an  export  tax  of  about  $12.25  per  ton.  In  the  past  30 
years  this  export  tax  has  netted  the  Chilean  Government  about 
$500,000,000.  Of  the  total  production  of  Chile  the  United 
States  imports  about  600,000  to  700,000  tons  per  annum  the 
balance  being  practically  all  shipped  to  European  countries. 
Chile  saltpeter  has  sold  as  high  as  $60  a  ton  but  since  1909  when 
the  agreement  among  the  producers  expired  the  price  has  ap- 
proximated $45  per  ton  f.o.b.  Liverpool,  making  a  price  of  from 
$35  to  $40  per  ton  f.o.b.  Chile. 

The  union  of  nitrogen  and  carbon  to  form  cyanides  and  with 
hydrogen  to  form  ammonia  are  two  of  the  earliest  forms  in  which 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  339 

the  combined  nitrogen  was  utilized.  Most  all  animal  and  vege- 
table refuse  contains  ammonia  compounds  and  these  were  the 
early  sources  of  ammonia,  and  animal  refuse  products  such  as 
hides,  hoofs  and  horns  were  the  sources  of  combined  carbon 
products  forming  the  cyanides.  Until  the  discovery  of  the  Mc- 
Arthur-Forrest  process  for  gold  extraction,  the  markets  for 
cyanides  were  comparatively  limited  and  there  was  no  great 
effort  made  to  produce  them  on  a  large  scale.  With  the  rapid 
development  of  this  art  in  the  recovery  of  the  low  grade  gold 
deposits  a  sudden  impetus  was  given  to  the  cyanide  industry, 
and  large  quantities  of  cyanides  are  now  manufactured  from 
ammonia  and  metallic  sodium.  Small  amounts  of  cyanides 
for  industrial  purposes  are  recovered  from  the  gas  retort  houses 
but  these  processes  are  not  generally  applied  and  no  particular 
effort  has  been  made  to  extend  the  processes  to  the  recovery 
of  cyanides  from  by-product  coke  ovens.  The  greater  portion 
of  the  cyanides  are  manufactured  in  England  and  Germany  and 
some  20,000  tons  per  annum  are  exported  annually  by  these  two 
countries.'  As  the  cyanides  of  sodium  and  potassium  for  gold 
recovery  purposes  sell  from  $300  to  $400  per  ton,,  they  represent 
one  of  the  highest  prices  of  nitrogen  directly  combined  with 
a  simple  element. 

The  third  great  commodity  of  commerce,  ammonia,  is  utilized 
extensively  in  industrial  arts  but  in  addition  has  been  used  for 
many  years  as  a  fertilizer.  The  annual  production  of  sulphate 
of  ammonia  now  amounts  to  about  1,250,000  tons  and  the  Liver- 
pool price  approximates  that  of  sodium  nitrate,  varying  from 
$45  to  $60  .per  ton.  Practically  all  of  this  sulphate  of  ammonia 
is  manufactured  from  coal  distillation  either  from  gas  house 
retorts  or  by-product  coke  ovens,  up  to  the  past  year  there 
having  been  practically  no  process  in  operation  for  the  direct 
synthesis  of  ammonia  from  its  compounds. 

All  the  older  retort  processes  for  the  manufacture  of  gas, 
recover  ammonia  by  washing  the  illuminating  gas  with  water. 
All  by-product  coke  ovens  likewise  treat  the  by-product  gas  for 
the  recovery  of  ammonia.  American  coals  run  from  0.9  per 
cent  to  1.4  per  cent  nitrogen  or  from  18  to  28  lb.  (8.1  to  12.7  kg.) 
of  nitrogen  per  ton  of  coal.  In  the  distillation  of  this  coal  about 
20  per  cent  of  the  nitrogen  is  recovered  from  the  gases  of  dis- 
tillation so  that  from  4|  to  7  lb.  (2.1  to  3.2  kg.)  of  ammonia  are 
recovered  per  ton  of  coal  distilled;  this  ammonia  when  united 
with  sulphuric  acid  forms  sulphate  of  ammonia,  giving  a  yield 


340  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

of  from  18  to  28  lb.  (8.1  to  12.7  kg.)  of  sulphate  of  ammonia  per 
ton  of  coal  distilled.  Weak  solutions  of  ammonia  water  are 
concentrated  from  the  gas  house  retorts  and  the  ammonia  dis- 
tilled from  this  water  by  breaking  down  the  ammonia  contents 
with  lime,  the  pure  ammonia  then  being  united  with  sulphuric 
acid.  In  many  of  the  coke  oven  plants  the  sulphate  is  formed 
directly  by  passing  the  gases  into  sulphuric  acid  forming  the 
ammonia  sulphate  by  a  direct  process. 

In  general  it  costs  about  $15  per  ton  of  ammonia  sulphate  to 
manufacture  the  sulphate  from  the  ammonia,  so  that  if  am- 
monia sulphate  is  selling  for  $45  per  ton,  $15  of  this  is  represented 
in  the  cost  of  sulphuric  acid  and  the  manufacturing,  making  the 
net  ammonia  cost  with  profit  $30  per  ton  of  sulphate  or  as  the 
nitrogen  content  of  the  sulphate  amounts  to  21  per  cent,  the 
nitrogen  represents  an  actual  value  of  7  cents  per  pound.  With 
the  great  increase  in  the  number  of  by-product  coke  ovens, 
there  has  been  a  greatly  increased  quantity  of  ammonia  sul- 
phate manufactured,  and  it  would  seem  that  under  these  condi- 
tions the  price  of  ammonia  sulphate  will  tend  to  diminish  rather 
than  to  increase.  The  actual  cost  to  the  by-product  coke  oven 
plants  recovering  the  ammonia,  in  addition  to  the  $15  for  manu- 
facturing the  sulphate  of  ammonia,  will  approximate  $10  per 
ton,  and  if  there  is  any  profit  to  be  obtained  from  the  sale  of 
ammonia,  they  can  afford  to  recover  it  at  this  figure. 

Another  source  of  ammonia  by  coal  distillation  is  from  producer 
gas  generated  on  what  is  known  as  the  Mond  system.  In 
this  process  steam  is  admitted  to  the  producer  in  excess,  so  that 
the  temperature  is  not  permitted  to  rise  to  a  point  where  the 
ammonia  liberated  by  the  fuel  is  decomposed.  This  excess  of 
steam  tends  to  protect  the  ammonia  and  it  is  recovered  from  the 
producer  gas  by  washing.  In  this  process  not  only  the  ammonia 
carried  in  the  volatile  products  is  recovered  but  also  a  large  per- 
centage which  ordinarily  remains  in  the  carbonaceous  residue 
of  the  coke  oven  and  gas  house  retort.  As  ordinarily  distilled 
about  50  per  cent  of  the  total  ammonia  of  the  coal  remains  in 
the  coke  residue  and  is  not  recovered.  In  the  producer  where 
this  coke  is  consumed  in  the  presence  of  steam  the  total  per- 
centage of  recovery  may  be  as  high  as  75  per  cent  of  the  theoretical 
nitrogen  contained  in  the  coal,  so  that  from  15  to  20  lb.  (6.8  to 
9.1  kg.)  of  nitrogen  may  be  recovered,  or  in  terms  of  ammonia 
sulphate,  from  60  to  80  lb.  (27.2  to  36.2  kg.)  of  ammonia  sul- 
phate may  be  obtained  per  ton  of  coal  consumed  in  the  producer. 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  341 

This  type  of  producer  has  not  been  extensively  utiHzed  in 
America  as  the  expense  of  installation  is  increased  by  the  neces- 
sity of  washing  a  very  large  volume  of  low  grade  gas,  the  volume 
of  gas  per  ton  of  coal  consumed  in  the  producer  being  about 
130,000  cu.  ft.  against  about  10,000  cu.  ft.  per  ton  of  coal  as 
distilled  in  the  coke  oven. 

A  number  of  these  plants  have  been  installed  in  England 
and  on  the  continent,  but  the  aggregate  of  the  ammonia  sul- 
phate produced  is  not  large  as  compared  to  that  from  coke 
ovens  and  gas  house  retorts. 

Available  Nitrogen  in  Commercial  Products.  The  question  of 
the  available  nitrogen  in  the  various  compounds  has  in  a  mea- 
sure determined  the  price  of  the  product,  the  utilization  in  the 
fertilizer  art  being  practically  the  basis  of  fixing  the  price. 
For  a  number  of  years  it  has  been  assumed  that  the  selling 
price  of  combined  nitrogen  would  be  from  12  cents  to  13  cents 
a  pound.  Thus  Chile  saltpeter  being  about  95  per  cent  pure 
nitrate  of  soda  would  have  a  theoretical  nitrogen  content  of 
about  16.5  per  cent  or  corrected  for  impurities  would  have  about 
15.5  per  cent  nitrogen. 

As  the  cyanides  until  recently  were  not  used  directly  in  the 
fertilizer  art  and  were  combined  with  more  expensive  products, 
their  price  has  not  been  regulated  by  their  content  of  combined 
nitrogen.  The  ammonia  used  in  the  fertilizer  art  is  almost 
entirely  used  as  sulphate  of  ammonia,  on  account  of  the  cheap- 
ness of  the  commercial  sulphuric  acid  and  the  ease  of  manu- 
facture, and  this  product  would  therefore  have  a  theoretical  con- 
tent of  21  per  cent  of  nitrogen. 

The  above  nitrogen  products  may  be  considered  the  funda- 
mental commercial  forms  in  which  combined  nitrogen  enters 
the  market,  and  while  the  fertilizer  industry  fixes  the  price  of 
combined  nitrogen,  it  is  only  one  of  the  many  industries  in  which 
vast  quantities  of  nitrogen  are  utilized.  Thus  about  50  per  cent 
of  all  the  Chile  saltpeter  imported  in  this  country  is  used  in  the 
manufacture  of  explosives,  while  an  additional  25  per  cent  is 
utilized  in  the  arts  requiring  nitric  acid.  Of  the  ammonia  sul- 
phate, a  very  large  percentage  is  used  directly  as  fertilizer, 
though  there  is  a  very  considerable  demand  for  use  in  chemical 
industries  and  such  commercial  applications  as  anhydrous  and 
aqueous  ammonia  used  in  the  refrigeration  art.  Practically 
all  explosives  have  utilized  nitrogen  compounds  as  a  principal 
ingredient.     The  earlier  black  gun  powder  having  used  Chile 


342  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

saltpeter,  charcoal  and  sulphur  and  the  later  so-called  smoke- 
less powder  utilizes  the  oxygen  carrying  property  of  nitrogen 
as  well  as  the  inherent  .molecular  energy  in  the  production  of 
such  high  explosives  as  nitroglycerin,  cordite,  lydite,  mellenite, 
gun  cotton  and  various  other  nitro-cellulose  compounds,  and 
modified  explosives  used  in  industrial  work,  such  as  dynamite 
and  various  blasting  powders. 

Fixation  Processes.  In  considering  the  fixation  of  atmos- 
pheric nitrogen  from  a  commercial  standpoint,  the  limitations 
will  be  imposed  by  the  selling  price  of  the  natural  product  from 
Chile,  covering  nitrate  compounds,  and  the  selling  price  of  am- 
monia sulphate  as  obtained  from  coal  distillation,  affected  as 
these  prices  will  be  by  the  manufacture  of  ammonia  from  at- 
mospheric nitrogen. 

In  competition  with  the  above  sources  of  nitrogen  there  has 
been  the  constant  effort  toward  the  fixation  or  rather  the  utiliza- 
tion of  some  of  the  vast  quantity  of  atmospheric  nitrogen  sur- 
rounding us. 

A  list  of  these  fixation  processes  would  contain  the  names  of 
hundreds  of  investigators,  and  from  the  past  twenty  years  of 
effort  there  may  develop  processes  which  at  present  are  still 
experimental;  but  of  the  various  processes  which  have  reached 
the  state  of  commercial  application  there  appear  to  be  four 
distinct  lines  of  development. 

First.  The  production  of  nitric  acid  directly  from  the  at- 
mosphere by  means  of  the  electric  arc.  In  this  process  the 
nitrogen  of  the  atmosphere  is  directly  combined  with  its  ac- 
companying oxygen  without  utilizing  any  other  chemical  sub- 
stances, the  process  consisting  essentially  of  a  powerful  arc  fur- 
nace through  which  air  is  forced,  causing  at  this  high  tempera- 
ture the  nitrogen  to  combine  with  the  oxygen  forming  nitric 
oxide,  NO. 

Second.  Methods  of  fixing  nitrogen  by  means  of  electric  fur- 
naces or  combustion  where  the  energy  of  chemical  combination  is 
utilized  causing  the  nitrogen  to  combine  with  some  substance  with 
which  there  is  a  pronounced  energy  of  chemical  combination. 
These  processes  include  furnaces  utilizing  calcium  carbide  with 
which  nitrogen  readily  combines  to  form  calcium  cyanamide 
CaCN2,  and  various  processes  for  making  combinations  of 
nitrogen  and  a  basic  or  alkaline  earth  metal  such  as  calcium 
nitride,  CasNa,  or  magnesium  nitride,  MG3N2,  or  aluminum 
nitride,  AIN,  the  chemical  action  usually  forming  a  nitride  or 
carbo-nitride. 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  343 

Third.  Processes  for  producing  ammonia,  NH3,  directly  from 
nitrogen  and  hydrogen.  These  include  the  effort  to  use  the 
various  forms  of  electric  discharge  by  which  the  nitrogen  mole- 
cule may  be  decomposed  and  in  the  presence  of  hydrogen,  form 
ammonia.  As  ammonia  decomposes  at  a  very  low  temperature 
(500  to  1000  deg.  cent.)  only  the  silent  discharge  seems  avail- 
able, and  the  yields  are  not  commercial.  The  most  promising 
of  all  direct  ammonia  processes  seems  to  be  that  of  Haber.  In 
this  process,  a  catalytic  agent  is  used  and  under  a  heavy  pressure 
the  nitrogen  molecule  is  decomposed  and  united  to  the  hydrogen 
thus  forming  ammonia.  Salts  of  uranium  seem  to  be  preferred 
as  the  catalytic  agent  and  have  the  power  of  acting  on  nitrogen 
at  a  temperature  of  about  500  deg.  cent. 

Fourth.  Production  of  a  high  temperature  by  combustion 
utilizing  either  catalytic  agents  or  simply  by  producing  a  high 
temperature  by  means  of  the  explosion  or  combustion  of  gases 
directly  combining  the  nitrogen  and  oxygen  to  form  nitric  oxide, 
NO.  This  method  early  used  by  Bunsen  in  the  combustion 
of  hydrogen  to  form  water  has  been  applied  to  coke  oven  gases 
by  Hausser.  A  bomb  is  used  and  the  mixture  of  gas  and  air  is 
fixed  electrically,  the  small  amount  of  NO  formed  is  recovered 
and  converted  into  nitric  acid,  HNO3. 

The  chemical  form  in  which  the  commercial  supplies  of 
combined  nitrogen  appear  on  the  market  is  due  largely  to 
existing  commercial  conditions.  The  nitric  acid  combined  as 
sodium  nitrate  occurs  in  this  form  simply  on  account  of  being 
'naturally  produced  in  this  form.  The  ammonia  appears  on  the 
market  as  ammonia  sulphate  largely  on  account  of  the  cheapness 
with  which  sulphuric  acid  can  be  obtained,  and  the  widely 
distributed  plants  for  its  manufacture,  making  it  one  of  the  cheap- 
est and  most  convenient  forms  of  combining  with  ammonia.  It 
is  probable  that  in  commercial  nitrogen  fixing  plants,  if  both 
ammonia  and  nitric  acid  are  manufactured,  one  of  the  most 
convenient  forms  for  marketing  this  product  will  be  by  using 
nitric  acid  in  place  of  sulphuric  acid  making  ammonia  nitrate, 
NO3NH4.  This  product  is  on  the  market  at  present  but  is  only 
manufactured  from  sodium  nitrate  and  from  ammonia,  or  in 
some  of  the  plants  where  nitric  acid  is  manufactured,  ammonia 
is  shipped  to  the  nitric  acid  plants  to  be  manufactured  into 
ammonia  nitrate.  The  advantage  of  ammonia  nitrate  is  that 
it  has  a  nitrogen  content  of  35%  in  this  respect  being  a  much  more 
concentrated  nitrogen  product  either  for  the  processes  of  manu- 


344  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

facturing  other  compounds  of  nitrogen  or  for  use  in  the  fertilizer 
industry. 

Physical  Limitations  and  Those  Fixed  by  Natural  Sources.  The 
competition  with  natural  sources  will  fix  the  commercial  limita- 
tions or  selling  prices  for  these  various  nitrogen  compounds  and 
in  considering  the  possible  developments  of  the  processes  it  will 
be  interesting  to  see  to  what  extent  they  have  definite  theoretical 
limitations,  as  these  will  greatly  affect  any  comparison  of  pos- 
sibilities. Before  considering  in  detail  these  processes  we  might 
endeavor  to  investigate  whether  our  present  conception  of  the 
physical  and  chemical  reactions  involved  impose  real  limitations, 
or  whether  there  is  an  uncertain  boundary  which  further  de- 
velopments may  encroach  upon,  perhaps  thus  continually  im- 
proving the  efficiency  and  posvsibilities  commercially.  If  for 
instance  the  nitric  oxide  processes  which  utilize  only  2  per  cent 
to  4  per  cent  of  the  energy  supplied  to  the  furnace  are  limited  to 
this  amount  by  the  inefficiency  of  the  apparatus,  there  is  much 
greater  possibilities  of  development  than  would  be  the  case  if 
the  process  has  definite  physical  or  thermodynamic  limitations, 
and  the  present  apparatus  utilizes  a  favorable  percentage  of  this 
possible  ultimate  limit.  To  some  extent  these  theoretical  limita- 
tions are  not  always  sharply  defined,  and  research  will  extend 
this  horizon,  but  we  may  determine  some  of  these  limitations 
quite  definitely. 

II.     Theoretical  Limitations 

As  we  are  considering  this  subject  from  its  engineering  aspects, 
it  may  be  excusable  to  examine  some  of  the  theoretical  limita- 
tions imposed  by  the  laws  of  physical  chemistry,  and  in  reviewing 
what  may  be  termed  elementary  formula,  it  is  interesting  to  note 
that  the  investigation  of  these  theoretical  limitations  has  been 
of  fundamental  importance  to  physical  chemistry  in  extending 
the  application  of  the  laws. of  chemical  dynamics. 

Molecular  Inertness  of  Nitrogen.  The  elements  carbon,  C, 
and  nitrogen,  N,  possess  a  marked  similarity  in  the  fact  that  the 
molecule  of  each  is  composed  of  two  or  more  atoms  united  to- 
gether with  a  bond  representing  a  large  amount  of  energy. 
Nitrogen,  having  an  atomic  weight  of  14,  has  a  normal  mole- 
cular weight  of  28,  indicating  two  atoms  to  the  molecule,  and 
in  this  molecular  form  it  occupies  79.2  per  cent  of  the  volume  of 
the  earth's  atmosphere.  To  separate  this  molecule  into  its 
constitutent  atoms  and  cause  these  atoms  to  combine  with  other 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  345 

elements  is  the  problem  of  the  fixation  of  nitrogen.  Unless 
combined  in  the  atomic  form,  the  enormous  bond  between  the 
atoms  causes  them  to  combine  upon  themselves  into  the  inert 
form  of  molecular  or  atmospheric  nitrogen.  The  ordinary  com- 
pounds of  nitrogen  are  formed  only  by  the  expenditure  of  a 
large  amount  of  energy,  the  union  of  molecular  nitrogen,  N2, 
and  molecular  oxygen,  O2,  to  form  nitric  oxide,  NO,  being  rep- 
resented by  the  formula, 

No  +  O2  =  2N0  =  -43,000  calories 

Or  in  other  words,  to  form  one  gram  molecule  of  nitric  oxide, 
NO,  requires  the  expenditure  of  energy  amounting  to  21,500 
calories. 

The  general  similarity  to  carbon  in  this  molecular  inertness 
makes    an    interesting    comparison. 
Thus 

C2  +  O2  =  2  CO  =  58,000  calories. 

2  CO  +  O2  =  2  CO2  =  134,000  calories. 

The  formation  of  one  gram  molecule  of  CO  therefore  represents 
the  liberation  of  29,000  calories  while  the  formation  of  one  gram 
molecule  of  CO2  from  CO  represents  67,000  calories  or  a  total 
of  96,000  calories  in  the  formation  of  CO 2  from  the  original 
elements  C  and  0.  When  amorphous  carbon  therefore  is  caused 
to  assume  the  gaseous  condition  and  unite  with  a  molecule  of 
oxygen  there  is  liberated  29,000  calories  but  after  assuming  this 
condition  in  which  the  molecule  is  no  longer  composed  of  the 
inert  carbon  molecule,  a  second  gram  molecule  of  oxygen 
unites  with  the  CO  and  hberates  67,000  calories  additional. 
The  second  molecule  of  oxygen  therefore  liberates  38,000  cal- 
ories more  than  the  first  molecule,  and  as  the  oxygen  molecules 
were  alike  this  energy  represents  the  bond  uniting  the  carbon 
atoms  and  the  energy  necessary  to  break  down  the  bond  between 
these  atoms  and  produce  a  gaseous  condition  from  the  amorphous 
condition. 

Returning  to  the  nitrogen  molecule,  it  is  apparent  that  the 
formation  of  the  gram  molecule  of  NO  requires  21,500  calories 
in  comparison  to  carbon  liberating  29,000  calories  to  form  CO; 
that  is,  the  nitric  oxide  reaction  is  endothermic  while  the  carbon 
monoxide  reaction  is  exothermic.  Upon  adding  a  second  mole- 
cule of  oxygen  to  the  nitric  oxide  to  form  the  peroxide  we  find, 

2  NO  +  O2  =  2  NO2  =  27,000  calories 


346  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

or  13,500  calories  per  gram  molecule  are  liberated  after  previously 
expending  21,500  calories  to  form  NO.  Since  there  is  liberated  only 
13,500  calories  upon  adding  a  second  molecule  of  oxygen,  the  net 
energy  required  to  form  the  NO2  would  be  8000  calories.  The  first 
molecule  of  oxygen  required  21,500  calories  whereas  the  second 
required  only  8000  calories,  so  that  13,500  calories  were  required 
by  the  nitrogen  molecule  to  prepare  it  for  combination  with  the 
oxygen. 

Dynamic  Equilibrium.  These  heats  of  combination  developed 
by  the  atoms  combining  upon  themselves  indicate  a  very  stable 
or  inert  molecule,  and  in  liberating  this  energy  to  assume  this 
more  stable  form  the  forces  exerted  are  of  large  magnitude. 
It  is  readily  apparent  from  this  for  instance  how  carbonaceous 
gases  can  readily  form  soot  and  cinders  and  other  amorphous 
forms,  when  the  union  with  oxygen  is  disturbed,  as  the  tendency 
of  the  atoms  to  unite  with  oxygen  or  to  form  carbon  molecules 
will  depend  upon  an  adjustment  of  the  surrounding  conditions. 
This  ever-changing  condition  of  equilibrium  constitutes  the 
dynamic  conception  of  equilibrium  displacing  the  static  equili- 
brium of  the  older  theories  of  chemistry.  In  order  to  more 
carefully  consider  some  of  the  theories  that  have  been  advanced 
it  may  be  of  interest  to  follow  further  some  of  the  concepts  of 
physical  chemistry.  The  fact  that  the  nitrogen  atom  has  this 
strong  tendency  to  combine  upon  itself  with  a  liberation  of 
energy  greater  than  the  combination  with  the  oxygen  atom, 
indicates  that  in  any  reaction  when  the  combination  with  oxygen 
has  made  possible  a  changing  of  the  atoms  there  will  be 
continuously  in  progress  an  action  and  a  reaction  and  the 
equilibrium  will  be  indicated  by  the  expression 

N2  +  O2  ^  2  NO 

the  sign  of  equality  being  displaced  by  the  two  arrows  indicating 
that  the  action  proceeds  in  each  direction,  that  is,  it  is  a  reversible 
reaction,  and  the  equilibrium  will  be  dependent  upon  condi- 
tions, the  two  most  important  conditions  being  temperature  and 
the  active  masses  of  the  substances  present.  Considering  first 
the  effect  of  the  active  masses  present,  if  it  be  assumed  that  the 
collision  of  molecules  causes  the  re-arrangement  of  the  atoms 
comprising  the  molecule,  and  that  of  these  collisions  only  a  cer- 
tain number  will  cause  the  re-arrangement  to  proceed  in  one 
direction,  the  re-arrangement    will   be   greater    the   more   fre- 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  347 

quently  the  collisions  take  place,  there  being  some  ratio  for  each 
individual  case,  and  the  collisions  possible  will  obviously  be  pro- 
portional to  the  concentration  present,  that  is,  the  collisions 
will  be  proportional  to  the  number  of  molecules  present.  With 
two  substances  present,  the  collisions  will  be  proportional  to 
the  molecules  of  each  present  and  hence  to  their  product. 

The  Velocity  Coefficient.  If  C\  and  Co  represent  the  special 
concentration  of  gram  molecules  present,  the  velocity  of  the 
reaction  will  be  proportional  to  CiCi  or  the  velocity  V  of  the 
reaction  will  be 

F  =  Ci  Cok 

where  ^  is  a  constant  or  coefificient  to  be  determined  for  the  given 
temperature.  This  velocity  of  reaction  will  proceed  each  way 
in  reversible  reactions,  the  concentrations  of  the  molecules  in 
the  reverse  action  being  represented  by  Cx  and  C-/  and  the 
velocity  of  reaction  by  V ,  the  velocity  of  the  reaction  in  the 
reverse  direction  will  be 

V'  =  C/  C.'k' 

where  k '  is  the  velocity  constant  to  be  determined  for  the  re- 
verse   reaction. 

The  Equilibrium  Constant.  The  chemical  driving  force  for 
any  reaction  will  continually  diminish  as  the  reaction  approaches 
equilibrium,  or  the  velocity  of  the  reaction  will  diminish  as  equilib- 
rium is  approached,  and  when  equilibrium  is  reached  V  will 
equal  V  and 

kClC2   =  k'C  C2' 

This  dynamic  equilibrium  indicates  that  as  much  reactive  sub- 
stance is  being  formed  in  a  given  time  as  is  being  decomposed, 
and  a  fixed  relation  therefore  results,  but  the  reactions  have  not 
ceased,  only  the  velocities  have  equalized,  and  the  driving  chem- 
ical forces  are  incapable  of  making  any  further  change  in  the 
reacting  substances  unless  the  velocity  in  one  direction  or  the 
other  is  changed.  In  this  state  of  equilibrium  the  ratio  between 
the  velocity  coefficients  or  constants  is 

-r  =      ^  ^     =  K  —  equilibrium  constant. 
k  C\Ci 

Or,  as  the  concentration  or  active  mass  in  any  reaction  increases, 
the  velocity  coefficient  increases,  and  for  each  change  in  equili- 


348  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

briuni  due  to  temperature  change,  there  is  a  definite  concentra- 
tion ratio  represented  by  this  equilibrium  constant  K. 

Partial  Pressures.  If  the  concentration  of  a  given  molecule 
is  C  and  the  collisions  of  the  molecule  are  proportional  to  this 
concentration,  if  there  are  two  molecules  the  collisions  between 
the  two  similar  molecules  will  be  C  times  as  great  as  one  molecule, 
or  C^.  As  the  total  pressure  of  a  mixture  of  gases  is  the  sum  of 
the  pressure  of  each  gas,  and  by  Avagadro's  hypothesis  the 
pressure  is  proportional  to  the  number  of  molecules  in  the  given 
space,  the  concentrations  instead  of  being  represented  by  gram 
molecules  C  may  be  expressed  as  partial  pressures  p,  and  equili- 
brium will  be  represented  by  the  ratio  of  the  partial  pressures  of 
the  gases.  Thus  two  molecules  of  NO  will  have  the  pressure 
p^o  while  ^N  and  po  may  represent  the  pressure  of  N  and  0. 
At  equilibrium  the  constant  K  will  then  become 


K 


pj^Po 

and  for  any  pressure  and  volume  the  familiar  equation 

PV  =  RT 

will  represent  the  work  done.  Where  P  is  the  pressure,  V  the 
volume,  R  the  gas  constant,  and  T  the  absolute  temperature. 
The  work  done  in  the  reaction 

will  consist  in  taking  one  molecule  of  N  from  the  pressure  p  and 
transferring  to  the  pressure  P  also  one  molecule  of  0  from  the 
pressure  p,  to  pressure  P,  and  offsetting  this  work  will  be  the 
transferring  of  two  molecules  of  NO  from  pressure  Pno  to  ^no 
when  the  pressure  of  a  gas  at  constant  volume  is  increased  by  the 
infinitely  small  amotmt  dp,  the  corresponding  work  done  dA  will 
be 

dA=  dpV 

The  Maximum  Work.     If  we  substitute  for  V  its  value  from 
the  equation 

pV  =  RT 
we  have 

p 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  349 

and 


and  for  the  work  done  between  the  limits  of  pressure  p  and  P  we 

have 

p 

A  =  RT    \  -^  =  RTln  4 


J 


P  ^      '    P 


where  In  is  the  natural  logarithm.     For  the  work  done  in  forming 
the  NO  at  a  temperature  T,  we  will  have 

N  +  O         -     2  NO 


A  =  RTln  -|^  +  RTln  -^  -  2  RTln  ^ 

-t    N  Po  Pi 


'no 

NO 


or  simplifying  and  assembling  the  initial  pressures  and  the  final 
pressures  in  separate  terms,  we  have  for  constant  temperature 

A  =  RTln  ^f/°    +  RTln 


P    NO  -t    N  -to 

But  the  first  term  represents  the  initial  pressures  or  the  work 
done  on  the  initial  condition  of  the  materials,  and  we  are  not 
called  upon  to  furnish  this  energy,  the  change  of  energy  being 
represented  only  by  the  second  term 


RTln 


-t    N-To 


p2 

This  quantity-^ — 5^is,  however,  the  ratio  of  pressures  or 

-t    N-i    O 

concentrations  represented  by  the  equilibrium  constant  K  and 
hence  the  work  done  at  any  given  temperature  may  be  repre- 
sented  by   the    equation 

A  =  RTlnK 

Vant  Hoff  has  applied  this  type  of  fundamental  equation  to 
a  wide  range  of  reactions  and  by  means  of  the  second  law  of 


350 


SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 


thermodynamics  has  made  it  applicable  to  temperature  and  con- 
centration changes  in  which  the  latent  energy  plays  an  important 
part,  for  in  these  reactions  the  product  of  the  specific  heat  by 
the  temperature  no  longer  represents  the  heat  transfer. 

Vant  Hoff's  Fundamental  Equation.  The  second  law  of  thermo- 
dynamics expresses  the  relation  of  A,  the  maximum  work  pos- 
sible at  a  temperature  T,  and  U,  the  decrease  in  energy  of  the 
system  in  relation  to  the  ratio  of  change  of  A  with  the  tem- 
perature T,  the  equation  being 


A-  U 


dA_ 
dT 


3500' 

*-"  3000° 
S3 

.^ 

^ 

,^ 

/ 

i 

|.2500° 

V 

^ 

/ 

^ 

1- 

§200^ 

< 

/ 

/ 

150d 

/ 

D- 

PERC 

2 

ENTAG 

EOF 

i 

'to.  X. 

i 

\ 

5 

Fig.  1 

d^ 
Substituting  in  this  the  value  of  A  and  -^  obtained  from  the 

equation  A  =  RTlnK 

we  have  when  both  A  and  InK  change  with  the  temperature   T 


dA_ 
dT 


RLnK  +  RT 


dlnK 
dT 


and  hence 


U  =  -RT- 


dhiK 
dT 


which  is  Vant  Hoff's  fundamental  equation  for  chemical  reac- 
tions in  which  the  heats  of  reaction  U  are  important  factors  and 
both   the   temperature    and   concentrations   may   be   variable. 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  351 

The  quantity  U,  which  in  thermodynamics  represents  the  de- 
crease in  energy  of  the  system,  may  here  represent  the  energy 
of  chemical  combination,  which  changes  very  little  with  changes 
of  temperature  and  is  generally  designated  as  Q,  and  in  the  case 
of  nitric  oxide  it  has  the  value  21,500  calories  per  gram  mole- 
cule; so  the  equation  becomes 

43,000  =  -i?r2.    '""^ 


dr 

and  when  using  the  air  as  a  source  of  nitrogen  in  which  the  nitro- 
gen content  is  79.2  per  cent  and  the  oxygen  21.8  per  cent  and 
letting  X  represe  nt  the  percentage  of  NO  formed,  as  |  x  will  be 
from  the  nitrogen  and  |  x  from  the  oxygen,  the  equilibrium  con- 
stant K  will  be 


(79-2  -  4)  (  20.8 -f) 


1811 

0.37 

1877 

0.42 

2023  bt.  0.52  and 

0.80 

2033 

0.64 

2195 

0.97 

2580 

2.05 

2675 

2.23 

Nernst's  Determinations  of  Equilibrium.  Nemst  and  his 
assistants  determined  x  for  a  number  of  temperatures,  the  cal- 
culated and  observed  values  being  as  follows: 

T.  X(Obs.)  X(Calc.)  Observer 

0.35  Nernst 

0.43  Jellinek 

0.64  Jellinek 

0.67  Nernst 

0.98  Nernst 

2 .  02  Nernst-Finckh 

2 .  35  Nernst-Finckh 

Nernst 's  calculations  of  x  or  equilibrium  volumes  in  per  cent  of 
NO,  using  air  at  temperatures  of  1500  deg.  T  to  3200  deg.  T 
are  plotted  in  Fig.  1. 

Rapidity  of  Dissociation.  Nernst  and  Jellinek  also  determined 
the  rapidity  with  which  the  NO  is  decomposed  or  dissociated 
at  the  various  temperatures  and  these  experiments  showed  that 
the  tendency  of  NO  to  revert  to  molecular  N  and  O  is  very 
slight  below  a  temperature  of  1500°  C  but  increases  very  rapidly 
with  the  temperature  so  that  the  time  of  withdrawing  the  pro- 
ducts through  the  varying  zones  of  heat  in  the  electric  arc  is 
sufficient    to    effect   a   large  amount  of  dissociation.     In  order 


352 


SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 


to  avoid  this  dissociation  a  rapid  movement  of  the  ait  through 
the  arc  or  the  arc  through  the  air  is  desirable,  and  this  in  turn 
causes  increased  radiation  and  convection  losses,  so  the  maxi- 
mum possible  temperature  of  the  arc  is  not  obtained  and  hence 
there  are  imposed  very  distinct  limitations  to  the  yield  of  NO. 

Haber's  Theory  of  Ionic  and  Electronic  Collisions.  Haber  and 
Koenig  investigated  the  possibility  of  utilizing  lower  temperatures 
in  the  arc  to  avoid  dissociation  by  enclosing  the  arc  in  a  water 
cooled  quartz  tube  whereby  moderate  temperatures  were  pre- 
served. In  place  of  the  molecular  collisions  we  have  assumed 
above  as  due  to  the  thermodynamic  condition  of  the  gas,  they  used 
a  vacuum  and  endeavored  to  utilize  the  kinetic  energy  from  the 
rapid  motion  of  the  ions  and  electrons  liberated  by  the  arc  stream 
under  these  conditions.  Habor  considered  it  possible  to  inc  rease 
the  thermodynamic  concentrations  about  50  per  cent.  His  tests 
indicating  that  using  a  temperature  of  3000  deg.  cent,  it  was 
possible  to  show  10  per  cent  concentrations  of  NO  which  would 
correspond  to  a  temperature  under  the  thermodynamic  equili- 
brium of  4300  deg.  cent.  Haber  gives  a  table  showing  the  effect 
of  various  mixtures  of  N  and  O  when  working  with  the  in- 
creased mean  free  path  of  the  molecules  due  to  a  vacuum  of 
100  mm.  of  mercury.  In  his  work  Haber  prefers  to  use  the 
square  root  of  the  equilibrium  constant  K  we  have  used  above, 
thus  enabling  the  partial  pressures  of  the  resulting  substances 
to  be  read  direct,  while  the  partial  pressures  of  the  ingredients 
are  expressed  as  square  roots  of  the  pressures. 

Haber's  table  for  a  pressure  of  100  mm.  is  as  follows: 


Gas  mixture 

K  = 
(NO) 

Thermodynamically 
calculated     absolute 
temperature  by 

O2  per  cent 

N2  per  cent 

percentNO 

(N2)i(02)i 

Air 

Half-half 

mixture 

Reversed  air 

mixture 

20.9 
48.9 
44.4 
75.0 
81.7 

79.1 
51.1 
55.6 
25.0 
18.3 

9.8 
14.4 
14.3 
12.8 
12.1 

0.284 
0.337 
0.337 
0.357 
0.397 

Haber 

Nernst 

4365 
4686 
4686 
4805 
5042 

4334 
4650 
4650 
4767 
5000 

The  yields  of  NO  per  kilowatt  hour  obtained  by  Haber  were 
unsatisfactory  and  the  complications  of  small  water  cooled  tubes 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  353 

and  working  under  a  vacuum  of  100  mm.  have  not  justified  com- 
mercially the  higher  concentrations  of  NO  he  obtained. 

Commercial  Processes  now  in  Use  have  Distinct  Limitations. 
We  may  assume  that  up  to  the  present  the  processes  in  com- 
mercial use  are  limited  strictly  by  the  thermodynamic  equili- 
brium of  the  Vant  Hoff  equation.  As  the  volume  of  gases  when 
working  with  low  concentrations  of  NO  are  considerable,  the 
radiation  and  convection  losses  as  well  as  the  transfer  of  sensible 
and  latent  heat  from  the  arc  to  the  gases  lower  very  materially 
the  temperature  of  the  arcs,  and  the  yields  therefore  indicate 
an  average  working  temperature  of  2200  deg.  cent,  to  2500  deg. 
cent.,  or  concentrations  of  1.5  percent  to  2  per  cent  NO  when 
working  with  air.  These  theoretical  limitations  of  the  direct 
process  of  forming  NO  have  therefore  led  to  many  efforts  to 
dissociate  the  nitrogen  molecule  by  other  means. 

Sources  of  Chemical  Energy.  Naturally  the  sources  of  chemical 
energy  have  offered  a  most  fruitful  field  but  like  the  synthesis 
of  carbon  compounds  a  considerable  elevation  of  temperature 
is  necessary  before  the  chemical  energy  becomes  effective  enough 
to  break  the  bond  of  the  nitrogen  molecule.  At  these  elevated 
temperatures  practically  all  elements  or  compounds  which  re- 
lease sufficient  energy  to  combine  with  nitrogen  have  a  greater 
combining  power  for  oxygen  so  the  processes  cannot  be  con- 
ducted with  air  but  involve  the  separation  of  the  nitrogen  from 
the  oxygen  of  the  air  as  a  preliminary  step.  The  compounds  of 
nitrogen  thus  formed  do  not  therefore  include  the  oxides  of 
nitrogen.  The  common  elements  exhibiting  the  most  pro- 
nounced tendency  to  combine  with  nitrogen  are  calcium  (Ca), 
magnesium  (Mg),  aluminum  (Al),  boron  (B)  etc.  The  carbides 
of  a  large  number  of  metals  also  exhibit  a  pronounced  tendency 
to  combine  with  molecular  nitrogen  when  heated. 

The  great  advantage  from  a  theoretical  standpoint  in  utiliz- 
ing these  processes  is  that  the  reaction  with  nitrogen  may  be 
made  more  complete  as  equilibrium  may  be  continually  dis- 
turbed by  withdrawing  the  compound  of  nitrogen  formed  or 
by  presenting  to  the  nitrogen  to  be  combined  fresh  combining 
surfaces  of  the  substance  and  the  action  may  be  caused  to  pro- 
ceed practically  quantitatively  thus  avoiding  heating  large 
quantities  of  materials  which  are  inert  to  the  reaction.  A  vast 
field  of  research  is  opened  by  these  possibilities  as  very  few  of 
the  equilibrium  figures  have  been  determined,  and  it  is  almost 
certain  that  direct  combustion  methods  may  eventually  be 
evolved  along  these  lines. 


354  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

The  experimental  data  are  so  meager  that  no  theoretical  limi- 
tations can  be  placed.  The  reactions  assume  unusual  importance 
however  on  account  of  a  wide  application  in  the  arts.  This  may 
be  best  illustrated  by  considering  the  formation  of  calcium 
carbide,  CaC2,  in  relation  to  its  three  reversible  reactions,  namely 

(1)  CaO  +  3Clr^  CaCa  +  CO 

(2)  CaC  ^Ca      +C 

(3)  CaO  +  C    ^  Ca      +  CO 

There  are  here  six  substances,  some  in  solid  form,  some  in  liquid 
and  some  gaseous  (molecular  and  atomic)  and  it  is  evident  that 
the  temperature  will  have  a  marked  influence  on  the  equilibrium 
which  will  exist,  and  the  reaction  will  be  greatly  affected  by  very 
minute  changes,  for  the  partial  pressure  of  the  gases  will  be  sud- 
denly changed  by  such  conditions  as  the  carbon  released  in  a 
gaseous  state  immediately  combining  to  form  amorphous  carbon, 
or  the  metallic  calcium  vapor  combining  with  the  oxygen  re- 
leased by  the  CO  to  form  calcium  oxide  which  immediately 
precipitates  as  a  solid.  The  fact  that  calcium  oxide  which  is 
most  refractory,  can  be  vaporized  at  a  temperature  of  1600  deg. 
cent,  to  1800  deg.  cent,  in  the  sense  that  the  calcium  is 
vaporized  and  decomposes  CO  to  again  precipitate  CaO 
is  one  of  the  actions  similar  to  the  fumes  in  smelting 
furnaces  which  accounts  for  a  heavy  loss  of  metal  at  tem- 
peratures not  ordinarily  capable  of  producing  fusion.  When 
nitrogen  is  inserted  in  a  reaction  of  this  kind,  there  are  im- 
mediately formed  complex  carbon-nitrogen  compounds,  but  the 
action  of  the  oxygen  present  is  to  dissociate  these,  allowing  the 
nitrogen  to  combine  into  the  molecular  form  and  the  metal  to 
precipitate  from  the  fume  as  a  minute  particle  of  metallic  oxide, 
the  carbon  precipitating  as  amorphous  carbon  in  the  form  of 
soot,  as  all  of  these  reactions  liberate  a  large  amount  of  energy. 
The  temperature  of  these  reactions  is  from  1500  deg.  cent,  to 
2000  deg.  cent,  and  is  therefore  well  within  the  range  of  combus- 
tion methods  if  the  combustion  could  be  in  contact  with  the 
substances  as  in  the  blast  furnace,  but  the  presence  of  the  oxygen 
necessary  for  combustion  prevents  the  formation  of  nitrogen 
compounds. 

The  effect  of  partial  pressures  in  these  reactions  is  of  funda- 
mental importance.     The  active  mass  of  a  solid  is  constant  and 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  355 

hence  at  the  boundary  surface  where  the  vSoHd  and  gases  meet 
there  is  a  high  velocity  of  reaction.  Equilibrium  will  be  pro- 
duced either  by  the  solid  forming  a  coating  of  the  compound  which 
will  place  it  in  equilibrium  or  by  the  pressure  of  the  gases  genera- 
ted from  the  reaction  producing  a  condition  of  equilibrium. 
Taking  the  familiar  calcium  carbonate  reaction  as  an  illustration, 

CaCog^CaO  +  CO2 

There  being  two  substances  in  the  solid  state  and  one  in  the 
gaseous,  the  equilibrium  constant  K  will  be 

^_        CaO  X  CO2 


CaCOg 

where  CaO  and  CaCOs  are  the  very  slight  vapor  pressures  of  the 
solids,  that  is  the  sublimation  pressures,  which  in  practice  are 
too  small  to  measure.  The  pressure  of  the  CO 2  will  be  practi- 
cally the  total  pressure,  and  as  this  varies,  the  velocity  constant 
K  will  vary  and  hence  for  any  temperature  there  is  but  one  pres- 
sure for  equilibrium.  LeChatelier  measured  the  temperatures 
and  pressures  of  the  above  reaction  over  a  wide  range  and  found 
a  variation  in  the  temperature  neceessary  to  produce  the  re- 
action of  from  547  deg.  cent,  at  27  mm.  pressure,  to  865  deg.  at 
1333  mm.,  or  in  other  words,  equilibrium  could  be  produced 
through  a  range  in  temperature  of  60  per  cent.,  and  at  one  point 
a  change  of  two  degrees  necessitated  a  change  in  pressure  of 
over  10  per  cent,  in  order  to  restore  equilibrium.  Rothmund 
found  the  equilibrium  pressure  of  CO  in  the  carbide  ot  calcium 
reaction  to  be  250  mm.  at  1620  deg.  cent.  If  the  CO  pressure 
was  raised  above  the  equilibrium  figure  at  this  temperature  CO 
was  absorbed  and  no  carbide  was  formed.  By  inserting  inert 
gases  so  the  CO  was  diluted  and  its  partial  pressure  reduced,  the 
temperature  of  the  formation  of  carbide  was  varied  over  20  per 
cenc.  These  results  all  indicate  that  for  the  nitrogen  reactions, 
the  actions  and  reactions  must  not  only  be  subject  to  accurate 
temperature  regulation  but  the  partial  pressures  must  be  con- 
trolled and  it  is  probable  that  definite  zones  of  reaction  must  be 
maintained.  The  present  commercial  applications  such  as  the 
Serpek  and  cyanamide  process  prepare  the  compounds  of  nitro- 
gen by  causing  the  nitrogen  to  react  largely  with  the  solid  mas- 
ses   and   thus  avoid  many   of  these   complications  due  to  the 


356  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

vapor  processes,  or  variable  minute  pressures  of  sublimation. 
By  substituting  the  resistance  furnace  for  the  arc  and  reducing 
the  metals  in  the  presence  of  nitrogen  thus  forming  nitrides  or 
forming  carbides  and  treating  these  in  the  presence  of  nitrogen, 
there  have  been  developed  a  number  of  processes  which  have 
been  commercially  applied. 

Ill— Description  op  Processes 
Three  processes  which  have  been  commercially  applied  for 
directly  preparing  nitric  acid  from  atmospheric  nitrogen  are 
first,  Birckland-Eyde ;  second,  the  Schonherr;  and  third,  the 
Pauling.  These  are  diagrammatically  shown  in  Figs.  2,  3,  and  5. 
Fundamentally  all  operate  on  the  same  principle  of  forcing  air 
into  intimate  contact  with  a  high-tension  arc  and  withdrawing 
the  [product  nitric  oxide,  NO,  as  directly  and  rapidly  as  possible 
in  order  to  reduce  the  amount  of  decomposition  of  the  resulting 
product.  As  these  processes  have  been  repeatedly  described 
in  detail  in  the  technical  press,  we  will  confine  our  attention  to 
general  comparisons. 

Birckland-Eyde  Process.  The  Birckland-Eyde  furnace,  il- 
lustrated in  Fig.  2,  has  been  the  most  extensively  used.  Its 
most  distinctive  features  are  the  use  of  the  magnets  A  which 
distort  the  arc  into  a  series  of  great  wheels  of  flame,  extending 
radially  outward  from  the  electrodes  E  located  normal  to  the 
paper  in  Fig.  2.  The  air  enters  through  the  conduit  C  and  is 
distributed  to  the  arc  through  the  holes  in  the  firebrick  lining. 
The  products  are  withdrawn  from  around  the  periphery  at  D. 
The  voltage  of  10,000  volts  is  reduced  by  an  inductive  reactance 
coil  to  about  5500  volts  across  the  electrodes.  The  alternating 
current  of  50  cycles  establishes  the  arc  across  the  ^/-shaped 
water  cooled  electrodes  E,  spaced  about  0.3  in.  (8  mm.)  apart 
and  a  flow  of  current  takes  place  across  this  ionized  path,  the 
electrons  formed  being  repelled  by  the  intense  magnet  field  of  the 
direct  current  magnets,  A,  and  their  discharge  radially  outward 
causes  the  arc  stream  to  follow  until  it  is  deflected  outward  in  a 
great  semicircle.  As  its  length  is  thus  increased  the  potential 
across  the  electrodes  rises  and  a  second  arc  is  established,  the 
effect  being  to  make  a  series  of  rapidly  expanding  arcs  which  are 
expanded  across  the  entering  air  spaces.  As  the  arcs  travel 
radially  outward  the  contact  of  the  ionized  arc  stream  with  the 
incoming  air  disrupts  the  nitrogen  molecule  and  causes  the 
formation  of  NO,  and  the  gaseous  products  travel  rapidly  to  the 


19151 


SUMMERS:  ATMOSPHERIC  NITROGEN 


357 


periphery  of  the  furnace  where  they  are  withdrawn  at  an  average 
temperature  of  about  1250  deg.  cent.  The  earHer  Eyde  fur- 
naces were  of  300  kw.  capacity  and  gave  a  concentration  of 
about  1.5  per  cent  NO  and  a  yield  of  about  500  kg.  of  nitric 
acid  per  kw.  per  year.  The  more  recent  furnaces  are  of  3000  kw. 
capacity  and  give  concentrations  of  about  two  per  cent  NO  and  a 
yield  of  580  to  600  kg.  of  nitric  acid  per  kw.  per  year,  or  65  to 
70  grams  of  nitric  acid  per  kw-hr. 

Schonherr  Process.     The  Schonherr  furnace  (Fig.  3)  consists 

G„  ^Water 


Water; 
Fig.  2 — Birckland-Eyde  Furnace 

A,  core;  B,  windings;  C,  gas  entrance;  D,  exit. 


CDC 

Fig.  3 — Schonherr  Furnace 


of  a  long  iron  pipe  4  having  an  electrode  E  inserted  in  the  bottom 
and  the  tube  itself  is  the  other  electrode,  the  distinctive  feature 
of  the  process  being  that  an  alternating  current  at  4500  to  5000 
volts  maintains  an  arc  of  from  23  to  25  feet  (7  m.  to  8  m.) 
The  furnace  is  started  by  forming  an  arc  from  the  lower  elec- 
trode to  the  wall  of  the  iron  pipe  by  means  of  a  lever  Z,  a  blast 
of  air  is  then  admitted  to  the  pipe,  whereupon  the  ionized  gases 
are  caused  to  ascend  and  carry  with  them  the  arc  stream.  In 
this  way  the  arc  is  caused  to  travel  toward  the  upper  end  of  the 


358 


SUMMERS:  ATMOSPHERIC  NITROGEN      [March  12 


tube  where  it  is  maintained.  In  practise,  the  air  stream  is  ad- 
mitted in  a  tangential  direction  causing  a  whirling  motion  to 
be  imparted  to  the  air  surrounding  the  arc,  this  creates  a  vortex 
motion  causing  the  arc  to  be  surrounded  by  the  cooler  air  and 
thus  protects  the  iron  pipe  which  is  wholly  unlined.  The  rapid 
passage  of  the  air  maintains  an  ionized  path  for  the  arc  stream 
and  the  arc  burns  quietly.  The  products  are  withdrawn  from 
the  upper  end  and  pass  downward  through  the  passes  /  to  the 
outlet  D.  Previous  to  being  withdrawn  they  are  cooled  by  the 
water  coooler  and  are  further  used  to  preheat  the  entering  air. 
The  temperature  of  the  exit  gases  is  about  850  deg.  cent. 
Provision  is  made  for  preheating  the  air  by  passing  it  upward  in 


Fig.  4 — Pauling  Nitric  Oxide  Furnace 


the  space  2  of  the  casing  and  then  downward  to  a  point  opposite 
the  electrode.  The  largest  furnaces  are  of  800  kw.  capacity 
and  maintain  an  arc  about  23  ft.  (7m.)  long.  They  give  an  NO  con- 
centration of  about  2.25  per  cent  and  a  yield  of  550  to  575  kg. 
per  kw-year  or  65  grams  per  kw-hr. 

Pauling  Process.  The  Pauling  furnace  (Fig.  4)  establishes 
an  a-c.  arc  of  4000  volts  between  two  curved  horns  much  after  the 
pattern  of  the  horn  type  lightning  arrester.  This  arc  when 
established  is  driven  upward  by  a  blast  of  air  admitted  at  B 
and  is  disrupted  by  the  diverging  horns.  A  sheet  of  arc  flame 
is  maintained  by  re-establishing  a  new  arc  as  the  previous  one 
is  elongated.  The  effect  is  to  create  an  arc  flame  about  30  in. 
(75  cm.)  high  and  to  have  this  flame  in  intimate  contact  with 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  359 

the  rapid  moving  air  used  to  blast  the  arc  flame.  Two  fur- 
naces are  usually  operated  together  and  to  assist  in  a  rapid  cool- 
ing of  the  products  a  portion  of  the  previously  heated  or  partially 
cooled  discharged  gases  are  admitted  to  the  top  of  the  furnace. 
The  arc  is  established  by  the  high  voltage  breaking  down  the  gap 
between  narrow  blades  C  located  in  the  horn  gaps,  and  as  these 
wear  away  they  are  continually  advanced  by  the  adjustments 
D.  The  percentage  of  NO  obtained  is  1.25  to  1.5  per  cent  in 
the  400  kw.  furnace  while  the  yields  are  525  to  540  kg.  per  kw- 
year  or  60  grams  per  kw-hr. 

Power  Factor  and  Electrode  Wear.  In  all  these  processes 
where  the  arc  is  distorted  the  power  factor  is  about  70  per  cent 
being  about  5  per  cent  lower  in  the  Schonherr  type,  apparently 
due  to  inductive  effects  of  the  iron  pipe  surrounding  the  arc. 
In  both  the  Pauling  and  Schonherr  furnaces  the  electrodes  arfe 
adjustable  and  the  air  blast  plays  directly  on  the  electrodes 
necessitating  this  adjustment  and  also  means  of  easy  renewals. 
The  Pauling  blades  last  less  than  24  hours,  the  Schonherr  elec- 
trode is  a  straight  rod  of  iron  and  is  fed  upward  as  it  burns 
away.  The  Eyde  water-cooled  copper  pipe  not  being  directly 
in  the  path  of  the  air  blast  lasts  three  to  four  weeks.  In  the 
operation  of  both  the  Eyde  and  Schonherr  furnaces  a  furnace 
is  placed  on  each  separate  leg  of  the  three-phase  circuit  so  that 
six  wires  are  used  for  each  generator.  In  the  installations  that 
have  been  made  the  furnaces  are  connected  direct  without 
transformers,  and  as  no  parallel  operation  of  generators  is  at- 
tempted a  large  number  of  cables  are  required  between  the  power 
house  and  the  furnaces. 

Efficiency  and  Losses.  It  will  be  noted  that  notwithstanding 
the  radically  different  types  of  these  furnaces  there  is  not  a  wide 
divergency  in  the  yields,  the  Schonherr  furnace  showing  the 
highest  concentration  of  NO  while  the  Eyde  probably  produces 
a  slightly  higher  output  per  kw-hr.  All  of  these  concentrations 
indicate  that  the  maximum  temperature  of  the  arc  is  not  utilized 
but  is  very  considerably  reduced  by  the  large  amount  of  air 
admitted.  If  the  temperature  of  the  arc  is  raised  by  admitting 
less  air  there  is  a  heavier  decomposition  of  the  products  and  all 
products  are  heated  to  a  higher  temperature  with  a  correspond- 
ing loss,  the  net  result  being  a  lower  yield  per  kw-hr.  As  the 
yield  per  kw-hr.  is  of  fundamental  importance,  adjustments  are 
governed  accordingly.  The  concentration  of  NO  effects  directly 
the  apparatus  for  recovering  the  products,  such  as  the  absorb- 


360  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

ing  towers,  and  the  lower  the  concentration  of  NO  the  greater 
the  losses  from  heating  the  inert  gases.  The  furnace  efficiency 
will  largely  be  determined  by  this  factor.  Let  us  take  for  ex- 
ample a  concentration  of  1.75  to  2  per  cent  NO  and  examine  the 
distribution  of  heat.  If  we  take  the  specific  heats  of  a  molecule 
of  the  diatomic  gases  as  constant,  there  will  be  no  difference  in 
the  specific  heats  per  molecule  of  N ,  O  or  NO,  and  taking  a 
standard  value  for  this  we  may  calculate  the  heat  energy  ex- 
pended. Let  us  assume,  following  Haber,  that  for  a  change  of 
temperature  /,  the  specific  heat  per  molecule  will  be 

6.8  +  0.0006  / 

The  gaseous  products  heated  in  the  furnace  with  L75  to  2 
per  cent  NO  concentrations  will  have,  from  Nernst  constants, 
an  absolute  temperature  of  about  2500  deg.  and  the  air  would 
have  an  initial  temperature  say  of  27  deg.  cent,  or  300  deg. 
absolute,  so  the  arc  would  raise  the  temperature  of  the  100 
molecules  through  2200  deg.,  or, 

sensible  heat  = 
100  (6.8  -f  0.0006/  X  2200deg.)  2200  deg.  =  1,786,400  calories. 

As  two  molecules  of  NO  require  a  latent  heat  of  formation  of 
43,000  calories,  the  total  heat  will  be  1,829,400  gram-calories; 
and  as  one  watt  hour  equals  860  gram-calories  the  energy  rep- 
resented will  be  equal  to  2.12  kw-hr.  for  two  gram-molecules 
of  NO.  Taking  the  atomic  weight  of  N  as  14  and  of  O  as  16, 
the  gram-molecule  of  NO  will  weigh  30  grams,  so  2.12  kw-hr. 
will  form  60  grams  of  NO. 

If  we  mix  this  NO  with  air  and  water  it  will  form  nitric  acid 
without  requiring  a  further  expenditure  of  energy,  thus, 

2  NO  +  li  0  +  H2O  =    2HNO3 

and  the  60  grams  of  NO  will  then  become  126  grams  of  HNO3; 
the  production  of  nitric  acid  will  then  be  126  grams  per  2.12 
kw-hr.  or  59.4  grams  of  HNO3  per  kw-hr.  Of  this  expenditure 
of  2.12  kw-hr.  the  formation  of  the  nitric  oxide  utilized 

=  2.35  per  cent. 


1,829,400 

and   the   sensible   heat   imparted   to   the    active  gases  to  raise 


19151  SUMMERS:  ATMOSPHERIC  NITROGEN  361 

their  temperature  to  form  two  molecules  of  NO  required  35,728 
calories,  so  this  represented 

35,728  ^  Q_ 

1.95  per  cent. 


1,829,400 


while  the  heating  of  inert  nitrogen  and  oxygen  or  that  portion 
of  the  air  which  was  not  utilized  but  which  was  heated  to  the 
furnace  temperature  was  represented  by 

1,750,672       ^^  ^ 

^^^329;^  =  95.7  per  cent. 

These  calculations  while  open  to  some  criticism  on  account  of 
the  uncertainty  of  the  figures  for  specific  heat  and  its  change 
with  temperature,  closely  approximate  the  conditions,  and  in- 
dicate that  low  concentrations  of  NO  when  formed  from  thermal 
reactions  are  extremely  wasteful.  If  concentrations  of  10  per 
cent  NO  are  obtained  with  temperatures  of  4200  deg.  to  4300  deg. 
cent,  the  yields  may  be  increased  to  135  to  140  grams  of  HNO3 
per  kw-hr.,  but  the  greatest  saving  in  energy  will  result  in 
utilizing  other  than  purely  thermal  e  nergy.  While  thermal 
energy  may  be  produced  more  cheaply  directly  from  fuel,  its 
temperature  possibilities  are  again  limited  by  the  inert  gases 
of  combustion  if  air  is  the  source  of  these. 

Hausser  has  commercially  applied  a  process  for  utilizing  coke 
oven  gases  by  means  of  an  explosion  bomb .  The  amounts  of  excess 
gases  may  be  limited  and  the  intimate  mixture  of  combustible 
and  oxygen  due  to  pre-compression  of  the  charge  permits  of  high 
combustion  temperatures  being  reached.  Fig.  5  shows  this 
bomb  having  a  capacity  of  1600  cu.  cm.  The  gases,  either 
illuminating  or  coke  oven  gases,  enter  through  the  inlet  after 
previously  exhausting  the  air  by  means  of  the  air  pump  outlet. 
Means  are  provided  at  A  for  injecting  under  high  pressure  a 
spray  of  water  to  cool  the  products  as  quickly  as  possible.  The 
ignition  takes  place  at  2  by  means  of  a  high-tension  spark  and  the 
explosion  is  propagated  outward  from  this  point  and  the  vapor 
condenses  on  the  enamel  lining  of  the  bomb. 

With  this  device  Hausser  obtained  a  temp  of  2100  T  and 
concentrations  of  NO  of  0.5  per  cent.  The  temperature  cal- 
culated from  the  assumed  figures  for  specific  heat  indicated  a 
concentration  of  0.3  per  cent  for  equilibrium  by  the  Nernst 
calculation  and  Hausser  sought  to  explain  this  increase  of  yield 


362 


SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 


as  due  to  a  chemical  reaction  induced  by  photo-chemical  effects 
similar  to  the  ionization  by  ultra-violet  or  actinic  rays  of  light. 
The  maximum  yields  were  99  grams  of  HNO3  per  cu.  m.  of  gas, 
or  equivalent  to  6.2  lb.  of  HNO3  per  1000  ft.  of  gas.  A  commer- 
cial plant  on  this  system  has  been  installed  in  Germany.  This 
low  concentration  of  NO  greatly  complicates  the  commercial 
application  as  the  absorbing  devices  are  more  cumbersome  and 
the  percentages  of  loss  are  higher. 

Method  of  Utilizing  NO.     All  of  the  above  nitric  acid  processes 
utilize    the   reaction 

2  NO  +  O2  ll^  2NO2 


GAS  INLET- 
BLAST  INLET-^^^'^s^;.?^^?^^     I^XSV^xyssy^TO  AIR  PUMP 


Fig.  5 — Hausser  Process 


which  is  an  exothermic  one  for  the  formation  of  the  peroxide, 
and  if  the  temperatures  are  controlled,  side  reacdons  can  be 
prevented  and  equilibrium  can  be  maintained  with  only  a  small 
percentage  of  NO  remaining.  The  gases  after  leaving  the  fur- 
naces are  usually  carried  through  waste  heat  boilers  where  50  to 
60  per  cent  of  the  heat  is  utilized  for  steam  production.  They 
are  then  cooled  in  aluminum  pipe  coolers  and  allowed  to  enter 
a  gas  holder  where  time  is  given  to  form  the  peroxide.  The 
products  then  enter  counter  current  absorption  towers  where 
the  reaction  with   water  forms 

2NO2  +  H2O  =  HNO3  +  HNO2 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  363 

the  nitrous  acid  HNO2  is  further  oxidized  and  utilized  to  form 
HNO3  in  contact  with  the  excess  oxygen  in  the  gases  and  with 
the  absorbing  water.  The  final  recovery  is  usually  made  by 
circulating  the  gases  through  two  towers  of  weak  alkaline  solu- 
tion, such  as  sodium  carbonate  and  this  is  converted  into  sodium 
nitrate  and  into  sodium  nitrite,  and  recovered  by  evaporation, the 
final  products  are  a  combined  nitrate-nitrite  of  sodium.  A  normal 
circulation  over  three  absorbing  towers  gives  an  acid  of  about 
30  per  cent  concentration  but  this  can  be  increased  to  45  or  50 
per  cent  by  recirculating  over  the  first  tower.  Further  concen- 
trations are  usually  made  by  evaporation.  All  of  these  processes 
are  simplified  by  increased  concentrations  of  NO  in  the  initial 
reaction.  About  two  to  three  per  cent  of  the  original  NO  is 
discharged  in  the  waste  gases  from  the  absorbing  towers.  It 
will  be  noted  that  one  great  advantage  of  these  processes  is  that 
they  require  the  handling  of  only  air,  gas  and  water  up  to  the  time 
the  nitric  acid  is  formed  in  the  absorbing  towers,  so  that  the 
simplest  handling  devices  suffice,  and  the  labor  is  a  minimum, 
also  no  chemicals  are  required  until  the  final  washing  with  the 
alkaline  solution  in  the  absorbing  towers,  and  this  may  be  a 
cheap  solution  such  as  lime  water  if  conditions  make  it  desirable. 

Cyanamide  Process.  The  very  low  yields,  representing  less 
than  5  per  cent  of  the  energy  expended  and  amounting  to  65 
kw-hr.  per  kg.  of  N  fixed,  naturally  have  turned  attention  to 
chemical  reactions  as  a  means  of  increasing  the  yields.  One  of 
the  most  important  of  these  is  the  process  for  making  cyanamide 
CaCN2.  As  a  separate  paper  is  to  be  presented  here  covering 
this  subject,  we  will  only  generalize. 

The  endothermic  reaction  and  the  heating  of  the  materials 
entering  into  the  calcium  carbide  reaction  have  an  approximate 
theoretical  value  of  3.1  kw-hr.  per  kg.  produced,  whereas  it 
requires  about  4  kw-hr.  to  produce  a  kg.  of  85  per  cent  calcium 
carbide  in  the  best  practise.  For  100  per  cent  carbide  it 
would  require  4.7  kw-hr.  and  the  efficiency  is  therefore, 

"       =  66  per  cent. 

The  union  of  carbide  and  nitrogen  is  exothermic  when  a  sufficient 
temperature  is  reached  so  the  actual  expenditure  of  energy  tor  this 
reaction  is  not  in  excess  of  0.1  to  0.2  kw-hr.  additional  for  the 
fixation  of  the  nitrogen.     We  require  further  the  preparation 


364  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

of  the  nitrogen,  and  the  grinding  of  the  carbide  to  prepare  it  for 
the  nitrogen  treatment.  The  cyanamide  can  be  used  directly 
in  the  fertiHzer  industry,  but  for  use  in  the  chemical  industries 
it  must  be  decomposed  to  form  ammonia,  or  if  nitric  acid  is 
required  it  must  be  made  from  the  ammonia  by  some  process 
such  as  the  Ostwald  contact  process.  We  may  figure  however,' 
that  the  yield  for  a  given  amount  of  electric  energy  which  amounts 
to  about  16.6  kw-hr.  per  kg  of  N  fixed,  is  from  four  to  five  times 
the  yield  of  the  direct  nitric  acid  processes,  while  offsetting  this 
is  the  cost  of  preparing  the  nitrogen,  the  cost  of  chemicals,  the 
handling  of  materials  at  high  temperature,  and  the  many  factors 
making  up  manufacturing  costs. 

The  Serpek  Process.  The  Serpek  process  is  typical  and  has 
been  quite  extensively  introduced  commercially.  The  reaction 
is  represented  by  the  equation 

AI2  O3  +  3C  +  N2  =  2A1N  +  SCO 

The  reacting  temperature  for  best  results  is  claimed  to  be  1800 
deg.  to  1900  deg.  cent.,  but  no  effort  is  made  to  define  the  equilib- 
rium conditions  and  it  is  very  evident  that  where  CO  enters  so 
actively  into  the  reaction  the  temperatures  can  be  materially 
altered  by  a  change  in  the  partial  pressures  of  the  N  and  CO. 
One  of  the  most  interesting  features  of  this  process  is  that  the 
impure  AI2O3  in  the  form  of  bauxite  is  fed  into  the  furnace  to- 
gether with  coal  and  the  sensible  heat  is  therefore  partially 
derived  from  the  coal.  Neglecting  the  specific  heats  of  the 
solids,  the  endothermie  reaction  requires  three  kw-hr.  per  kg.  of 
aluminum  nitride,  having  an  approximate  content  of  26  to  34 
per  cent  N;  it  would  require  therefore  9  to  10  kw-hr.,  per  kg.  of 
N  under  the  best  conditions  if  the  coal  and  pioducer  gas  were 
capable  of  supplying  all  the  heat  energy  required  to  produce  the 
required  temperature  in  the  gaseous  and  solid  products.  In  the 
case  of  cyanamide  it  requires  about  4  kg.  of  high  grade  carbide 
per  kg.  of  nitrogen  or  a  kg.  of  N  requires  16  kw-hr.  under  favor- 
able conditions  and  about  0.2  kw-hr.  for  heating  the  carbide 
against  10  to  12  kw-hr.  in  the  Serpek  process.  If  all  energy  were 
supplied  from  the  electric  source  the  Serpek  process  would  require 
practically  the  same  electrical  energy  as  the  cyanamide  process. 
There  is  a  distinct  advantage  in  being  able  to  use  producer 
gas  in  place  of  preparing  purified  nitrogen  and  there  is  a  further 
advantage  in  conducting  the  process  with  one  operation.     In 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  365 

practise  Serpek  uses  a  revolving  barrel  furnace  of  the  resistance 
type  the  resistance  consisting  of  a  squirrel-cage  construction  which 
continually  agitates  the  material  as  it  passes  through.  In 
all  nitride  reactions  this  is  essential,  as  the  materials  become  coated 
with  a  covering  which  protects  the  interior  and  prevents  further 
absorption  of  nitiogen.  Serpek  feeds  bauxite  and  coal  from  a 
producer  type  of  furnace  into  this  revolving  electric  furnace 
and  the  sensible  heat  is  thus  utilized  to  heat  the  material  as  it 
travels  to  the  electric  furnace  the  product  is  discharged  from  the 
electric  furnace  as  aluminum  nitride  with  a  content  of  26  per 
cent  to  34  per  cent  N. 

The  aluminum  nitride  can  be  treated  with  steam  and  the  N 
converted  into  ammonia  or  the  ammonia  may  be  converted  into 
nitric  acid.  Serpek  claims  a  process  for  converting  the  nitride 
directly  into  nitric  acid  but  no  details  are  available.  One  draw- 
back to  using  Bauxite  is  that  the  resulting  aluminum  oxide  is 
more  difficult  to  use  than  the  impure  bauxite  and  the  process 
does  not  work  as  economically,  it  is  probable  that  some  cheap 
catalytic  agents  may  be  found  to  substitute  for  the  bauxite  but 
otherwise  the  Serpek  process  should  necessarily  find  its  greatest 
application   in    connection   with    the   reduction    of   aluminum. 

Haber- Catalytic  Process.  One  other  process  which  has  at- 
tracted marked  attention  on  account  of  the  scientific  eminence 
of  its  inventor  as  well  as  the  commercial  results  obtained  is  the 
Haber  process  for  the  synthesis  of  ammonia  directly  from  its 
components  N  and  H.  This  means  that  there  must  be  a  supply 
of  these  elements  available  or  they  must  be  cheaply  produced. 
The  reaction  is  an  exothennic  one  producing  11,000  calories  per 
gram -molecule  so  the  problem  is  not  so  much  the  energy  consump- 
tion as  the  peculiarities  of  the  reaction 

N2  +  3H2  =  2NH3 

The  ammonia  formed  is  practically  decomposed  at  750  deg.  cent, 
it  has  been  difficult  to  get  any  substance  to  react  with  the 
N  at  this  low  temperature,  and  while  the  nitrogen  was  made 
active  toward  many  substances  it  was  easy  to  decompose  the 
resulting  NH3  into  its  constituent  molecules  and  all  yields  ob- 
tained were  too  low  to  justify  commercial  results.  Haber's 
success  seems  due  more  to  ingenuity  in  constructing  his  appara- 
tus and  to  the  discovery  of  a  suitable  catalyzer  than  to  any  de- 
parture from  previously  known  principles.     The  fact  that  one 


366  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

molecule  of  N  and  three  of  H  form  only  two  molecules  of  am- 
monia indicates  that  the  volume  occupied  by  the  ammonia  will 
occupy  only  one-half  the  space  occupied  by  its  constituent  gases 
and  hence  this  contraction  of  volume  will  be  assisted  by  pressure. 
Haber  increased  the  pressure  on  the  reacting  gases  to  200  atmos- 
pheres, and  as  a  catalyzer  he  used  uranium.  He  found  at  500 
deg.  cent,  he  could  react  on  the  nitrogen  and  upwards  of  8  per 
cent  of  ammonia  could  be  formed  before  equilibrium  took  place. 
By  using  limited  amounts  of  N  and  an  excess  of  H  the  equilibrium 
pressures  were  adjusted  well  within  the  decomposition  limits 
of  temperatures  and  by  withdrawing  the  gases  from  the  catalyzer 
as  they  reached  equilibrium  the  process  was  made  continuous. 
The  fact  that  decomposing  ammonia  creates  a  most  destructive 
corrosive  agent  had  to  be  met  and  the  retorts  had  to  be  made 
strong  enough  to  stand  the  effects  of  the  high  pressure,  and  also 
possible  explosions,  as  hydrogen  compressed  to  200  atmospheres 
and  heated  to  500  deg.  cent,  is  a  most  active  agent  in  the  pres- 
sence  of  impurities  such  as  oxygen,  sulphur,  etc.  The  retorts 
can  be  made  of  very  moderate  size,  and  a  number  of  them  used 
and  by  heating  internally  with  electric  resistance  the  shells  are 
not  subject  to  the  effects  of  temperature,  so  the  process  seems 
to  have  met  with  very  pronounced  technical  success.  The 
consumption  of  energy  for  heating  the  gases  is  very  slight  as 
the  exothermic  reaction  will  compensate  largely  for  the  heat 
required  to  release  this. 

The  preparation  of  the  N  and  H  and  the  compressions  to  200 
atmospheres  will  represent  the  greatest  costs  of  production. 
It  will  be  noted  that  all  products  are  handled  in  the  gaseous 
condition,  being  most  favorable  for  low  labor  costs.  The  am- 
monia is  extracted  from  the  mixture  of  N  and  H  by  slightly 
cooling  the  gases  until  the  point  of  liquefaction  of  ammonia 
is  reached  and  the  ammonia  condenses  out.  The  remaining 
gases  are  passed  back  to  the  retort  without  sacrificing  the  orig- 
inal pressure  of  compression.  The  work  done  on  the  gases  is 
thus  reduced  to  a  minimum  and  equilibrium  can  be  continually 
disturbed  by  withdrawing  the  products  without  heavy  losses 
This  process  involves  the  expenditure  of  approximately  1.5 
kw-hr.  per  kg.  of  N  and  therefore  represents  the  lowest  consump- 
tion of  energy  of  any  of  the  fixation  processes. 

From  an  engineering  point  of  view  the  various  processes  must 
be  considered  from  other  than  the  technical  standpoint,  the 
question  of  particular  application  being  the  guiding  considera- 
tion in  most  cases. 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  367 

IV — The  Economics  of  Nitrogen  Fixation 
Fertilizers.  In  considering  nitrogen  fixation  and  its  relation 
to  fertilizers  we  must  remember  it  is  only  one  of  the  three  im- 
portant ingredients  of  fertilizers,  the  other  two  being  phosphor- 
ous and  potassium.  It  is  possible  to  obtain  nitrogen  from  the 
atmosphere  and  transfer  it  to  the  soil  by  means  of  nitrifying 
bacteria  which  may  be  cultivated  by  such  plant  life  as  the  le- 
gumes, to  which  family  belong  the  clover  and  alfalfa.  These 
plants  have  a  nodule  on  the  stem  which  is  the  seat  of  the  bacteria 
activity  and  if  the  plants  containing  this  nitrogen  are  returned 
to  the  soil  the  soil  may  be  enriched  in  nitrogen,  but  the  crop 
must  be  sacrificed  or  partially  so.  When  it  is  not  desirable  to 
plant  these  nitrifying  crops  recourse  must  be  had  to  nitrogen 
in  the  form  of  fertilizer.  Unfortunately  all  crops  deprive  the 
soil  of  fertility,  and  in  the  case  of  phosphorous  and  potassium 
converted  into  the  crops,  these  must  actually  be  replaced,  or 
barren  soil  will  eventually  result.  Each  soil  must  be  treated 
for  the  particular  crop  it  is  to  bear  and  usually  there  are  fixed 
mixtures  which  become  standard  for  various  crops.  These  mix- 
tures contain  the  nitrogen  the  phosphorous  and  the  potassium 
in  varying  amounts.  The  output  of  nitrogen  from  a  chemical 
works  would  ordinarily  be  shipped  to  these  fertilizer  manufac- 
turers unless  the  chemical  works  desired  to  manufacture  the 
mixed  fertilizers. 

Of  all  the  processes  we  have  considered  the  cyanamide  is  the 
only  one  which  manufactures  a  product  which  is  in  a  form  to  go 
into  the  fertilizer  market  direct.  The  nitric  acid  processes  must 
unite  the  acid  with  some  alkaline  base  such  as  sodium,  lime  or 
ammonia  and  the  ammonia  processes  must  unite  the  product 
with  an  acid  such  as  sulphuric  or  nitric.  The  nitride  processes 
can  hardly  afford  to  ship  the  nitride,  as  it  is  combined  with  an  ore 
or  base  such  as  aluminum  oxide  which  may  be  more  desirable 
in  the  aluminum  industry,  as  the  fertilizer  industry  will  pay 
only  on  a  nitrogen  basis. 

Prices  of  Nitrogen.  In  general  the  price  of  combined  nitro- 
gen as  we  have  seen,  is  fixed  by  the  price  of  Chile  nitrate.  Thus 
if  this  sells  for  two  cents  per  lb.  and  contains  15  per  cent  N  the 
price  per  lb.  of  N  is  13.2  cents  and  this  in  turn  would  make 
the  price  of  ammonia  sulphate  having  21  per  cent  of  nitrogen 
2.7  cents  per  lb.  These  have  been  current  prices.  In  consider- 
ing the  production  of  nitrogen  products,  it  would  seem  that 
while  these  prices  control  nitrogen  for  the  fertilizer  industry, 


368  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

it  would  be  desirable  to  produce  if  possible  products  which  are 
manufactured  from  these  crude  products,  and  thus  avoid  com- 
petition with  the  natural  products  direct.  Nitric  acid  of  com- 
merce is  manufactured  from  soda  nitrate  by  treatment  with 
sulphuric  acid,  about  72  per  cent  ot  the  sodium  nitrate  being 
nitric  acid.  As  the  by-products  of  this  operation  only  partially 
pay  the  costs,  the  manufacturing  costs  leave  the  nitric  acid 
with  a  value  of  50  per  cent  over  the  value  as  nitrate.  Hence  a 
chemical  works  could  afford  to  produce  nitric  acid  when  they 
could  not  afford  to  add  a  manufacturing  cost  to  produce  a  fer- 
tilizer from  the  nitric  acid  and  then  sell  it  in  compeation  with 
the  crude  Chile  nitrate. 

A  large  portion  of  the  phosphate  rock  of  this  country  is  treai:ed 
with  sulphuric  acid  to  form  the  so-called  super  phosphates. 
If  nitric  acid  is  used  in  place  of  sulphuric  acid  the  super  phos- 
phate can  be  produced  at  the  same  bime  as  a  fertilizer  of  lime 
nitrate,  or  if  pretened  a  high  concentration  of  phosphoric  acid 
can  be  produced  from  lower  grade  phosphate  rock.  Industries 
of  this  kind  promise  more  favorable  results  commercially  than 
does  the  direct  pi  eduction  of  fertilizer.  If  low  grade  products 
are  manufactured  at  close  prices  a  very  laige  volume  of  business 
is  a  necessity  and  works  of  this  character  and  magnitude  are 
more  apt  to  be  a  result  of  successful  development  rather  than 
an  initial  venture  in  a  field  beset  with  uncertainties  as  to  the 
profits  and  the  chances  of  a  development  of  other  processes 
reducing  these  if  they  are  problematical. 

Costs  of  Making  Products.  Let  us  consider  more  in  detail 
some  of  the  costs.  We  may  assume  approximately  that  the 
labor  and  repairs  in  furnace  room  and  absorbing  tower  will 
cost  $10  per  ton  of  nitric  acid  and  if  nitric  acid  is  selling  for 
$60  per  ton,  we  have  a  margin  for  power  cost,  interest,  general 
expense,  etc.  of  $50,  and  if  we  produce  '500  kg.  of  acid  per  kilo- 
watt year  it  will  require  two  kw-yr.  per  metiic  ton  or  $25  per 
kilowatt-year,  and  we  must  absorb  all  inteiest  charges  and 
general  expense  in  this.  If  the  yield  can  be  made  550  kg.  per 
kilowatt  year  and  we  can  sell  for  $60  per  short  ton,  we  will 
require  1.8  kw-yr.  or  $28  per  kw-yr.  We  must  assume  that 
the  product  does  not  have  to  be  packed  for  shipment  and  that 
there  are  no  selling  costs  involved,  and  we  must  figure  on  an 
output  so  that  our  units  may  be  large  enough  to  bring  the  in- 
vestment in  plant  down  to  $80  pei  ton  of  acid  so  the  annual 
charge  may  be  $8  or  net  $20  per  kilowatt-year,  and  if  $5  are 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  369 

allowed  for  general  expense  the  best  we  can  do  will  be  about 
$15.00  per  kilowatt-year. 

We  see  it  would  be  hopeless  to  attempt  to  put  this  acid  into 
a  product  to  compete  with  the  fertilizer  prices,  for  they  are  some 
50  per  cent  lower  in  selling  price,  and  it  will  involve  a  cost  tor 
some  raw  material  to  mix  with  the  acid,  the  cost  of  manufacture 
and  a  packing  and  shipping  charge.  We  must  then  abandon 
any  idea  of  making  fertilizer  from  nitric  acid  prepared  by  the 
direct  oxidation  of  atmospheric  nitrogen  in  the  electric  arc  until 
such  time  as  we  can  improve  the  very  low  efficiency  due  to  the 
thermodynamic  limitations  of  the  reaction.  We  can  only  hope 
to  utilize  this  process  in  the  manufacture  of  nitric  acid  coupled 
with  some  other  product  which  will  procure  for  it  a  higher  price. 
There  is  for  example  a  limited  demand  for  the  nitrite  of  sodium 
NaN02  used  in  the  dye  industry  and  this  is  manufactured  by 
reducing  nitric  acid  with  molten  lead  thereby  adding  another 
manufacturing  operation  to  the  acid  cost.  This  nitrite  may 
be  cheaply  made  by  taking  a  mixture  of  NO  and  NO2  such  as 
we  would  have  in  parts  of  the  system  and  passing  it  into  water 
or  sodium  hydi  oxide,  thus 

NO2  +  NO  -f  H2O  =  2  HNO2 
or, 

4N0  +  2  NaOH  =  N2O  +  2  NaNOg  +  H2O 

and  this  process  would  produce  a  product  selling  for  four  to 
five  cents  per  lb.  We  must  remember,  however,  that  the  price 
of  nitric  acid  is  not  a  fixture  and  that  a  cheap  combined  nitro- 
gen fertilizer  will  cut  the  price  of  sodium  nitrate  and  hence  the 
price  of  nitric  acid.  We  are  confronted  then  with  the  fact  that 
all  processes  will  be  affected  by  the  success  of  any  one  process 
that  is  a  large  enough  success  to  affect  the  market  conditions 
of  combined  nitrogen  and  upset  the  ruling  prices  in  the  fertilizer 
industry.  It  is  useless  to  look  only  to  cheap  power  as  a  solution 
of  this  problem  as  the  real  solution  is  in  the  improvement  of 
processes. 

Let  us  roughly  compare  the  power  requirements  ot  the  pio- 
cesses  as  we  have  outlined  them  above  and  we  find 

Direct  oxidizing  of  atmospheric  nitrogen  5  per 
cent,  efficiency,  yield  at  550  kg.  per  kw-year, 
requires  per  kg.  of  N 65  kw-hr. 

Cyanamide  process  66  per  cent,  efficiency  in  carbide 
1  per  cent,  loss  in  heating  to  combine  with  N.,  re- 
quires per  kg.  of  N, ., 16.6     " 

Also  preparation  of  N. 


370  SUMMERS:  ATMOSPHERIC  NITROGEN     [March  12 

Aluminum  nitride  using  coal  to  heat  products  to 

temperature  of  reaction  requires  per  kg.  of  N 12     kw-hr. 

Catalytic  method  of  combining  N  and  H  to  form 

ammonia,  requires  per  kg  of   N 1.5     " 

Also  preparation  N  and  H,  refrigeration,  and  com- 
pression to  200  atmospheres. 

The  general  tendency  abroad  in  figuring  the  cost  of  water 
power  is  to  give  only  the  operating  costs  and  from  this  one 
sees  costs  of  producing  power  figured  at  from  50  cents  to  SI. 00 
per  kilowatt  year  and  in  using  these  figures  erroneous  ideas 
have  been  widely  circtilated.  In  this  country  it  has  been  stand- 
ard practice  to  consider  the  investment  in  the  power  plant, 
that  is,  the  cost  of  the  development  and  the  property,  as  fixing 
very  largely  the  cost  of  producing  the  power.  It  is  quite  com- 
mon to  have  labor  and  supplies  cost  not  more  than  one  dollar 
per  kilowatt-yeai ,  but  this  would  not  be  considered  as  represent- 
ing the  cost  of  the  power.  Where  a  chemical  industry  owns 
the  hydroelectric  power  plant  as  well  as  the  chemical  works, 
the  foreign  practice  is  inclined  to  consider  the  investment  as  a 
whole  and  apportion  the  costs  of  operation  to  the  various  de- 
partments, while  interest  on  the  capital  is  charged  to  the  profits. 
The  costs  of  producing  power  are  therefore  uniformly  much  lower 
than  we  are  accustomed  to  figure  on.  There  are  many  plants 
in  operation  in  the  chemical  industries  abroad  whose  real  costs 
of  producing  power  are  no  lower  than  many  of  the  more  favored 
locations  in  this  country. 

Off-Peak  Loads.  One  of  the  chief  interests  in  the  chemical 
utilization  of  electrical  energy  is,  centered  in  the  possibilities 
of  off-peak  or  off-season  loads,  as  American  plants  generally 
have  a  certain  proportion  of  power  which  can  be  disposed  of  to 
better  advantage  than  selling  the  entire  output  as  low  priced 
power  to  chemical  industries.  This  off-peak  power  is  difficult 
to  utilize  in  furnace  work,  where  the  cooling  of  the  furnace  and 
its  charge  is  an  important  factor  both  from  the  standpoint  of 
cost  and  of  output,  and  again,  adjustments  may  be  so  disturbed 
from  an  interrupted  output  as  to  be  absolutely  impracticable. 
One  of  the  possible  solutions  for  off  peak  utilization  appears  to  be 
in  the  adoption  of  some  system  where  fuel  is  also  utilized  and 
the  radiation  losses  are  not  excessive  under  conditions  of  banked 
fire  when  the  electric  portion  of  the  heat  energy  is  not  in  use. 
Some  of  these  combination  processes  may  promise  a  solution 
of  the  off-peak  load  situation  more  attractive  than  the  straight 


1915]  SUMMERS:  ATMOSPHERIC  NITROGEN  371 

electric  furnace,  which  is  difficult  to  cool  down  entirely  without 
unduly  affecting  the  conditions.  In  all  plants  the  volume  of 
output  is  the  determining  factor  in  absorbing  the  general  over- 
head charges,  and  any  intermittance  must  diminish  output  with 
its  accompanying  disadvantages. 

If  the  furnace  could  be  operated  on  the  off-season  load  and  its 
product  stored,  and  the  chemical  works  utilizing  this  product 
could  operate  on  a  normal  schedule,  this  would  form  one  solu- 
tion ;  or  another  would  be  if  by  chance  the  off-season  power  were 
available  at  the  time  the  greatest  demands  were  to  be  met,  such 
as  preparing  a  fertilizef  product  at  the  season  of  fall  rains  for 
the  early  spring  delivery.  All  of  these  plans  however  suggest 
the  necessity  of  operating  at  least  a  portion  of  the  plant  continu- 
ously in  order  to  meet  fixed  charges  and  preserve  an  operating 
organization.  If  the  chemical  works  requires  a  moderate  amount 
of  power  for  its  processes  in  year  around  operation  and  only  its 
surplus  for  manufacturing  its  crude  material  at  the  off  season 
peak  it  promises  the  greatest  possibilities. 

The  future  of  nitrogen  fixation  is  alluring  and  promises,  many 
developments  along  lines  other  than  those  we  have  considered, 
but  already  the  market  has  felt  the  effects  of  these  various  pro- 
cesses and  instead  of  nitrogen  being  figured  at  thirteen  cents  per 
pound,  it  is  confidently  predicted  it  will  very  shortly  find  its 
level  at  a  selling  price  of  about  eight  cents,  making  a  cost  of  pro- 
duction of  five  to  six  cents  per  pound  and  thus  reducing  sodium 
nitrate  to  about  1.33  cents  per  pound,  or  approximately  $30  per 
long  ton,  and  the  lower  grade  mines  will  feel  this  and  be  forced 
to  curtail. 

It  therefore  seems  very  certain  that  before  25  years  shall  have 
elapsed  since  Sir  Wm.  Crook's  made  his  memorable  address, 
the  Chile  nitrate  beds  will  have  vastly  curtailed  their  production, 
not  from  exhaustion  but  from  the  inroads  made  by  the  onward 
advance  of  chemical  and  electrochemical  engineering. 


liiiiiiiiiW 


.^::ii 


liiiiiiilil 

i    iii! « 


lllillliil  iiiUiUJinmiii 


